As is well known to those skilled in the art, various methods for forming a plastic product are employed depending upon the shape and the material of the product. Typical methods for forming a plastic product include a blow-molding method. In the blow-molding method, a parison or a sheet made of plastic is softened by being heated due to its thermoplastic characteristic and is positioned between split molds. Then, compressed air is supplied into the parison or sheet, and the parison or sheet is inflated against the surfaces of the molds. In this state, by cooling the parison or sheet, the manufacture of a hollow plastic product is completed. That is to say, in the conventional blow-molding method, a thermoplastic plastic material which is heated and softened is extruded and preformed in the shape of a tube to form a parison. Then, the parison is placed between molds which are adopted for the blow-molding method, and is heated and softened. Thereupon, by blowing air into the parison, an end product is obtained.
Meanwhile, many shapes of air spoilers have been used in conformity with the kinds of vehicles. The air spoilers for vehicles function to prevent vehicle bodies from being lifted under the influence of air currents created when the vehicles travel at high speeds, thereby ensuring driving stability at high speeds.
An air spoiler and a manufacturing method thereof as disclosed in U.S. Pat. Nos. 6,531,020 and 6,293,613 will be described below. As shown in FIGS. 1A and 1B, an air spoiler 10 for a vehicle is installed at the rear end of a vehicle body 12 to extend in a widthwise direction of a trunk lid 11. A plurality of fastening members 20 are integrally coupled to the lower surface of the spoiler so as to fasten the spoiler 10 to the upper surface of the trunk lid 11 in a state in which the spoiler 10 is separated from the upper surface of the trunk lid 11 by a predetermined distance.
Each fastening member 20 has a cup-shaped fastening member body 21 which has a predetermined height, and a flange 22 which is formed integrally with the upper end of the fastening member body 21 to be fused to the outer surface of a hollow spoiler body. The bottom wall of the fastening member body 21 is defined with a plurality of vent holes 23 for discharging air existing in the fastening member 20 to the outside when the fastening member 20 is fused with the spoiler body. A locking bolt 24 for fastening the fastening member 20 to the trunk lid 11 of the vehicle is secured to the bottom wall of the fastening member body 21.
The spoiler 10 is manufactured using the blow-molding method as described above. When manufacturing the spoiler 10 using the blow-molding method, the fastening member 20 is fused with the outer surface of a hollow parison which is a preform of the spoiler body.
FIGS. 2A and 2B are schematic cross-sectional views illustrating a procedure for manufacturing the spoiler for a vehicle using the blow-molding method according to the conventional art. In particular, a method for fusing the fastening member to the hollow parison will be described with reference to these drawings.
First, an upper mold 30 and a lower mold 40 each of which is defined with a cavity 50 to form a predetermined shape of a spoiler by the blow-molding method are prepared. The upper surface of the lower mold 40 which defines the cavity 50 is also defined with a plurality of seating grooves 41 into which the fastening members 20 are inserted, respectively. An air discharge passage 42 is defined at the bottom of each seating groove 41 to discharge air existing in the fastening member 20 to the outside.
After the fastening member 20 is seated in the seating groove 41, the hollow parison 60 is placed between the upper mold 30 and the lower mold 40, and the upper mold 30 is moved downward to enclose the hollow parison 60. Then, as shown in FIG. 2B, with the upper and lower molds 30 and 40 enclosing the hollow parison 60, compressed hot air is blown into the parison 60. Thereupon, the parison 60 is fused by the blown hot air and is inflated against the surfaces of the molds 30 and 40 which define the cavities 50, whereby a spoiler body 13 having a predetermined shape is formed. At the same time with this, a portion of the parison 60 which is overlapped with each fastening member 20 is inflated into the fastening member 20 and is fused to the inner surface of the fastening member 20. At this time, the air existing in the fastening member 20 is discharged to the outside through the plurality of vent holes 23 which are defined in the bottom wall of the fastening member 20.
Finally, as the molds 30 and 40 are cooled, the parison 60 is gradually hardened, and the spoiler 10 for a vehicle is produced in a state in which the plurality of fastening members 20 are integrally coupled to the outer surface of the spoiler body 13 having the predetermined shape.
However, in the fastening member 20 molded as described above, in the case that the distance measured between the spoiler 10 and the trunk lid 11 is substantial, that is, the height of the fastening member 20 is substantial as shown in FIGS. 3A and 3B, the material of the parison 60 is not uniformly fused with the fastening member 20 along the height of the fastening member 20 but can be gradually decreased in its thickness along the height of the fastening member 20 or can be exhausted at any point on the height of the fastening member 20.
If the fastening member 20 is molded in this way and the material of the parison 60 is not uniformly fused with the fastening member 20 along the height of the fastening member 20, the strength of the fastening member 20 fused with the spoiler 10 is degraded, and the fastening member 20 is likely to be broken after the spoiler 10 is fastened to the trunk lid 11.
Also, due to the fact that the material of the parison 60 fused to the fastening member 20 cannot have a uniform thickness, durability of the fastening member 20 is decreased, and vehicle-driving stability may be deteriorated.